1. Field of the Invention
The present invention relates to solar cells. More specifically, the present invention relates to solar cells fabricated using CVD epitaxial Si films on metallurgical-grade (MG) Si wafers.
2. Related Art
The negative environmental impact caused by the use of fossil fuels and its rising cost have resulted in a dire need for cleaner, cheaper alternative energy sources. Among different forms of alternative energy sources, solar power has been favored for its cleanness and wide availability.
A solar cell converts light into electricity using the photoelectric effect. There are several basic solar cell structures, including a single p-n junction, p-i-n/n-i-p, and multi-junction. A typical single p-n junction structure includes a p-type doped layer and an n-type doped layer of similar material. A hetero-junction structure includes at least two layers of materials of different bandgaps. A p-i-n/n-i-p structure includes a p-type doped layer, an n-type doped layer, and an optional intrinsic (undoped) semiconductor layer (the i-layer) sandwiched between the p-layer and the n-layer. A multi-junction structure includes multiple semiconductor layers of different bandgaps stacked on top of one another.
In a solar cell, light is absorbed near the p-n junction. The resulting carries diffuse into the p-n junction and are separated by the built-in electric field, thus producing an electrical current across the device and external circuitry. An important metric in determining a solar cell's quality is its energy-conversion efficiency, which is defined as the ratio between power converted (from absorbed light to an electrical energy) and power collected when the solar cell is connected to an electrical circuit.
Materials that can be used to construct solar cells include amorphous silicon (a-Si), polycrystalline (poly-Si), crystalline silicon (crystalline Si), cadmium telluride (CdTe), etc. FIG. 1 illustrates an exemplary solar cell based on a crystalline-Si wafer. Solar cell 100 includes a crystalline-Si substrate 102, a p-type doped single-crystal Si layer 104, an n+ silicon emitter layer 106, a front electrode 108, and an Al back electrode 110. Arrows in FIG. 1 indicate incident sunlight.
Based on industrial surveys, crystalline-Si-wafer based solar cells dominate nearly 90% of the market. However, the cost of conventional solar grade Si is well above $100/kg, which drives the cost of solar cells to $3-$4 per Watt peak (Wp). To lower the cost, various methods have been explored to utilize cheaper and lower grade Si for solar cell manufacture. Due to its cheap price, metallurgical-grade (MG) Si has been considered for making solar cells. The purity of MG-Si is usually between 98% and 99.99%. Impurities in the MG-Si include metals, boron, and phosphorus. To meet the purity requirement for high-efficiency solar cells, MG-Si needs to be purified.
U.S. Pat. No. 4,193,975 describes a method to purify MG-Si by melting Si with Al and silica slag followed by directional cooling. European Patent EP1958923A1, as well as US Patent Applications 2007/0128099A1, 2007/0202029A1, 2005/0053539A1, and 2008/0178793A1, also describe metallurgical methods for purifying Si. However, the quality of purified metallurgical Si wafers does not match that of solar-grade polysilicon, and the performance of the fabricated solar cell is less stable.
In U.S. Pat. No. 7,175,706, Mitzutani et al. of Canon describe a method for forming a high-purity polycrystalline Si thin film on MG multi-crystalline Si substrates and a corresponding method for fabricating solar cells. However, the multi-crystalline MG-Si substrate tends to produce multi-crystalline Si thin film, which results in a lower solar cell efficiency. On the other hand, T. H. Wang et al. propose a method that utilizes a liquid phase growth method for growing a high-purity silicon layer on an MG-Si substrate (see Solar Cell Materials and Solar Cells, vol. 41-42(1996), p. 19-30). Although a high-performance solar cell is demonstrated, the cost of liquid phase growth is prohibitively high, thus hindering any possible commercial application. Moreover, in U.S. Pat. No. 5,785,769, Ciszek proposes using an MG-Si substrate for deposition of crystalline thin-film Si. However, the boron concentration in the Ciszek MG-Si substrate is too high to form a high-efficiency solar cell.